The present invention provides a method of making a plurality of masks of a color cathode ray tube (CRT), of the type which are opposed to the phosphor screen of a CRT at a small distance therefrom and are separated from each other, each aperture of each mask being arranged coaxially with a corresponding aperture of another mask over the entire area of the masks.
One such CRT having this type of mask structure is the mask-focusing color picture tube. In a mask-focusing color picture tube, different potentials are applied to the masks and an electrostatic lens is formed between the facing masks. The electron beam utility factor is significantly increased compared with a convential shadow mask type color CRT. A mask-focusing color picture tube is described in Japanese Utility Model publication No. 38930/1972, and U.S. Pat. Nos. 2,971,117 and 3,398,309.
Another type of CRT which has the above described mask structure is described in Japanese Patent Publication No. 2698/1980. This color CRT has two masks. One mask acts as a color selection electrode and the other mask acts as a electron shield for preventing the other mask from being bombarded by electron beams and from being deformed by its rising temperature resulting from that bombardment.
In the both types of color CRTs, the corresponding apertures of the masks must be aligned coaxially with the electron beams. However, it is difficult to make or assemble a plurality of masks with such high precision. In a conventional manner, each apertured flat mask is pressed into its desired curved shape independently from the other mask. The masks are made of thin metal plates and have a relatively large area so that they are subject to being deformed during handling in the manufacturing process. The curvature of each mask is inevitably slightly different from that of the other masks at a given position on the masks. Therefore it is difficult to precisely align the corresponding apertures of each mask.